1. Field of the Invention
This invention relates to a drilling method and to drilling apparatus. In particular, aspects of the invention relate to combined bore drilling and bore isolation methods and apparatus.
2. Description of the Related Art
In oil and gas exploration and production operations, subsurface hydrocarbon-bearing formations are accessed by drilling bores from the surface to intersect with the formations. Drilling is accomplished using a drill bit mounted on the end of a drill support member, commonly known as a drill string. The drill string may be rotated via a top drive or rotary table on a surface platform or rig, or a downhole motor may be mounted towards the lower end of the string. The drilled bores are lined with steel tubing, known as xe2x80x9ccasingxe2x80x9d, which casing is cemented in the bore by filling the annulus between the casing and the surrounding bore wall with cement slurry. The casing inter alia supports the bore wall and prevents fluid flowing into or from the bore through the bore wall.
During a drilling operation it is normally the case that the drill string passes through an upper section of the bore, which is cased, and. a lower and more recently drilled bore section which is uncased. While drilling, it is not uncommon for the bore to intersect formations which create difficulties for the drilling operator, including: unstable formations which collapse into the bore; swelling formations which restrict the. bore and may trap the drill string in the bore; porous formations which result in loss of returning drilling fluid; and fluid-containing formations which result in uncontrolled flow of gas or liquid into the bore.
In some cases these difficulties may be overcome by, for example, pumping specialised fluids downhole to treat the problem formation. However, in other cases it may be necessary to retrieve the drill string and then run in casing or other bore liner to isolate the problem formation before drilling may recommence. Clearly, these operations will be time consuming and incur significant extra expense. Further, in the event of significant immediate problems, it may even become necessary to abandon the well.
In normal drilling operations1 the sequence of events in drilling and then casing a bore is similar, that is following drilling to a desired depth the drill string is retrieved and a casing string is then made up and run into the bore.
It is among the objectives of embodiments of the present invention to provide a method and apparatus which permit bore drilling and bore isolation operations to be executed in a single xe2x80x9ctripxe2x80x9d, that is a drill string need not be retrieved and a separate casing string run in prior to a bore lining or isolation operation being carried out.
According to the present invention there is provided a drilling method comprising: mounting a drill bit on a drill string including a section of expandable tubing; providing a tubing expander in the string; advancing the drill string through a bore; passing the expander through the expandable tubing to expand the tubing; and retrieving the drill bit from the bore, through the expanded tubing.
According to another aspect of the present invention there is provided drilling apparatus comprising: a drill string including a section of expandable tubing; a drill bit mounted on the string; and a tubing expander mounted on the string, whereby the expander is operable to expand the expandable tubing downhole such that the drill bit may be retrieved through the expanded tubing.
Thus, the invention allows a section of tubing to be expanded downhole to, for example, isolate a problem formation, and the drill bit to then be retrieved through the expanded tubing. In addition, in directional drilling, other equipment such as bent subs, motors and MWD apparatus will be mounted on the string and could also be retrieved through the expanded tubing. As the expandable tubing forms part of the drill string, conveniently forming the lowermost section of the drill string, the tubing may be put in place relatively quickly, as there is no requirement to retrieve the drill string and then run in a separate string of bore liner. The invention may also be utilised to drill and line a section of bore, which may not necessarily contain a problem formation, in a single trip. In such applications there may be occasions, for example, when the bore is not to be extended further, when the drill bit may not need to be retrieved and may be left in the sump of the bore.
The expanded tubing may be cemented in the bore.
The drill bit may be a bi-centre bit or a retractable or collapsible bit, to facilitate retrieval of the bit through the expanded tubing, and also to facilitate the drilling of relatively large bores below existing casing.
When drilling below a cased section of bore it is preferred that the length of the expandable tubing section is selected to be greater than the length of the uncased section of bore, such that there is an overlap between the existing casing and the expandable tubing; the expandable tubing may be expanded at the overlap to engage the casing, and thus create a hanger for the expanded tubing. In other embodiments the expandable tubing may be otherwise located or secured in the bore.
Preferably, the expandable tubing forms the lower section of the drill string and a drill assembly, which may consist solely of the drill bit, but which may also include directional drilling apparatus, such as bent subs, motors and MWDs, is mounted to the lower end of the expandable tubing section.
Preferably, the tubing expander is initially located in an upper part of the expandable tubing, and is advanced downwards through the tubing to expand the tubing. Most preferably, the expander and the drill bit define corresponding profiles such that, following expansion of the tubing, the expander may engage the bit and allow the bit to be retrieved with the expander. Preferably also, the coupling between the expander and the drill bit is such that there may be a transfer of torque therebetween, allowing further drilling of the bore with the drill bit coupled to the expander; this may be useful to allow expansion of the lowermost part of the expandable tubing and drilling of a pocket beyond the end of the section of bore lined with the expanded tubing.
Preferably, the expandable tubing is deformed by compressive plastic deformation or yield of the tubing, with a localised reduction in wall thickness resulting in a subsequent increase in tubing diameter. Most preferably, the deformation is achieved by rolling expansion, that is an expander member is rotated within the tubing with a face in rolling contact with an internal face of the tubing.
Preferably, the tubing expander comprises a body and one or more rolling expander members mounted on the body. The one or more expander members may be radially extendable, or may be inclined to the tubing axis to define an expansion cone. To expand the tubing, the expander is rotated and advanced through the tubing. The tubing expander may comprise a plurality of expanding sections, and in the preferred embodiment two expanding sections are provided, a first section including a plurality of rollers in a conical configuration, and a second section in which the roller axes are substantially parallel to the tubing axis. The first section may provide a degree of initial deformation by a combination of compressive and circumferential yield, while the second section may provide a subsequent degree of deformation substantially by compressive yield. Other forms of expanders may be utilised, such as a. fixed cone or expansion mandrel, however the expansion mechanism of a fixed cone, that is substantially solely by circumferential yield, is such that the axial forces required to advance such a cone through expanding tubing are significantly greater than those required to advance a rolling expander through expanding tubing.
The tubing expander may be rotated from surface, or may be rotated by a downhole motor mounted to the string.
Preferably, the tubing expander is releasably axially and rotatably lockable relative to the expandable tubing, and thus may form the coupling between the expandable tubing and the remainder of the drill string. When it is desired to expand the tubing, the expander may be rotatably unlocked from the tubing. Preferably, this follows an initial deformation of a first portion of the tubing into engagement with existing casing to create an initial lock against rotation of the tubing relative to the surrounding casing. The expander is then rotated relative to the tubing to create at least a portion of a tubing hanger. The expander may then be axially unlocked to allow the expander to advance through the tubing. The lock against relative location may be provided by couplings between the expander and the tubing which are released on initial deformation of the tubing, and the axial lock may be provided via a releasable swivel.
In other embodiments it may be necessary or desirable to retain a small annulus between the expandable tubing and the casing. This allows the expanded tubing to be cemented and sealed using conventional means. Further, sufficient initial torque resistance may be provided by the expandable tubing to allow the rotary expander to initiate rotary expansion before there is any contact between the tubing and the casing; for example a ball may be dropped to allow actuation of a release tool between the expander end the tubing.
The advancement of the tubing expander through the tubing may be achieved by application of weight, or alternatively or in addition may be achieved or assisted by provision of a suitable tractor arrangement, as described in WO93/24728, the disclosure of which is incorporated herein by reference. Such a tractor may include a plurality of rollers having skewed axes of rotation such that rotation of the tractor, with the rollers in contact with the surrounding tubing, produces an axial driving force. The rollers nay be urged radially outwardly, by mechanical or preferably fluid pressure force, to grip the tubing and such that the tractor may also provide for a degree of expansion of the tubing.
The expandable tubing may take any suitable form, and may be solid wall tubing, slotted or otherwise perforated tubing, or may incorporate sections of sand screen or the like. If the expanded tubing is to serve to isolate problem formations then clearly solid tubing will be preferred. The tubing may be provided with a seal arrangement, such as an elastomeric coating at the lower end thereof. Such an arrangement may be useful in situations where drilling fluid losses are being experienced to a formation that has been previously drilled. Losses could be mitigated by such a seal arrangement and would permit removal of the bit under safer well control conditions.
The drill string may take any appropriate form, and may he formed from drill pipe or from a reeled support, such as coiled tubing.
The expandable tubing may be expanded to a diameter close to the diameter of the drilled bore, and may be expanded such that the tubing contacts the bore wall.
According no a further aspect of the present invention there is provided a drilling method comprising mounting a drill bit on a drill string including a section of expandable tubing; providing a tubing expander In the string; advancing the drill string through a bore; and passing the expander through the expandable tubing to expand the tubing by compressive yield.
According to a still further aspect of the present invention there is provided drilling apparatus comprising: a drill string including a section of expandable tubing; a drill bit mounted on the string; and a tubing expander mounted on the string, the expander having at least one rolling expander member, whereby the expander is operable to expand the expandable tubing downhole by rolling expansion to produce compressive yield.